1. Field
Various features relate to methods of electromagnetic interference (EMI) enclosures, and in particular to EMI enclosures for package on package (PoP) radio frequency (RF) integrated circuit devices.
2. Background
Package on package (PoP) is an integrated circuit packaging method that combines vertically discrete logic and memory ball grid array (BGA) packages into one unit. Two or more packages are installed atop each other (i.e. stacked) with a standard interface to route signals between them. This allows higher component density in devices, such as mobile phones, laptop computers, and digital cameras.
PoPs that contain radio frequency (RF) components, such as RF amplifiers and other RF active and/or passive components (e.g., filters, duplexers, etc.) may require electromagnetic interference (EMI) shielding (also commonly referred to as RF shielding) in order to isolate the RF components from the surrounding environment. This shielding prevents the PoP's RF components from leaking out RF energy to the surrounding environment, and also prevents unwanted, extraneous RF signal noise of the environment from being injected into the RF PoP.
FIGS. 1 and 2 illustrate an EMI shield 100 found in the prior art. Specifically, FIG. 1 illustrates a top perspective view of the EMI shield 100, and FIG. 2 illustrates a bottom perspective view of the shield 100. The EMI shield 100 is typically made of a metal, such as aluminum, copper, etc. The shield 100 is sized to fit over one or more RF integrated circuits, such as RF PoP circuits. Once in place the shield 100 acts as a Faraday cage and insulates the RF circuitry within from RF radiation leaking into or out of the protected circuitry. The EMI shield 100 may feature a plurality of holes 102 that are sized small enough to still block RF radiation that has wavelengths significantly larger than the diameter of the holes.
FIG. 3 illustrates a schematic block diagram of a PoP circuit 300 found in the prior art that is covered with an EMI shield 302. The PoP circuit 300 includes a first package substrate 304 and a second package substrate 306. The second package substrate 306 is stacked on top of the first package substrate 304. The first substrate 304 may include at least one integrated circuit (IC), such as an RF power amplifier IC 308. The second substrate 306 may include a plurality if ICs 310, such as one or more passive duplexers and/or filters (e.g., surface acoustic wave (SAW) filters). The ICs 308, 310 are each electrically and physically coupled to their respective substrates 304, 306 through a plurality of soldering bumps 312. The second substrate 306 is electrically and physically coupled to the first substrate 304 through one or more soldering balls 314 or conductive pillars.
The PoP circuit 300 has a couple significant disadvantages. First, the plurality of ICs 310 coupled to the second substrate 306 have poor thermal conductive paths, which cause the heat generated by the ICs 310 to build up in the PoP circuit 300 and degrade performance. For example, a majority of the heat generated by the second substrate's ICs 310 are dissipated only through the soldering balls/pillars 314, which are located near the edges of the second substrate 306 and are relatively few in number. Thus, even though the second substrate's ICs 310 may be passive ICs (e.g., passive filters) that generate a fraction (e.g., ⅛th) of the heat of the high power, active RF power amplifier IC 308, the poor thermal conductive paths 314 coupled to the second substrate's ICs 310 causes these ICs 310 to reach undesirably high temperatures. By contrast, the first substrate's IC 308 has relatively good thermal conductive paths that allow the relatively high amounts of heat energy generated by the RF power amplifier 308 to be dissipated away. These thermal conductive paths include thermal vias 316 located within the first substrate 304 that electrically and thermally couple the RF power amplifier 308 to soldering balls 318 and/or heat spreaders that help dissipate heat.
Second, the location and limited number of soldering balls 314 that electrically couple the second substrate's ICs 310 to the first substrate 304 (e.g., ground and power nets) also limit the electrical performance of the PoP circuit 300. The soldering balls 314 cause a bottleneck that particularly increases the parasitic inductance between the second substrate's ICs 310 and the ground/power nets. This inductance reduces the electrical performance of the ICs 310 (e.g., SAW filters).
The EMI shield 302 is made of metal and is designed in such a way to act as a Faraday cage that fits over the plurality of RF devices 308, 310. Although the EMI shield 302 prevents a substantial amount of undesirable RF radiation to leak out from or into the PoP circuit, it does nothing to alleviate the two aforementioned problems above. Thus, there exists a need for improved package-on-package designs featuring EMI shields that—in addition to providing protection for RF radiation—help improve the thermal and electrical performance of underlying integrated circuits within the package-on-package devices.